1. Field of the Invention
This invention relates to a method for sealing a ceramic molding, especially a silicon ceramic molding, for hot isostatic pressing. In the hot isostatic pressing (HIP) process, porous mouldings of a silicon ceramic material are compressed under high pressure acting all around and under high temperature. Hot isostatic pressing reduces the porosity of the moulding and so improves its mechanical strength at elevated temperatures as well as its resistance to oxidation. The pressures used in hot isostatic pressing run in the vicinity of 3000 bars, the temperatures at about 1750.degree. C. Use is normally made of a gas as a medium to transfer the pressure. In order to keep this gas from penetrating into the pores of the molding, the latter requires encapsulating.
2. Discussion of the Prior Art
Encapsulation is currently being practiced such that the ceramic molding is enclosed in an evacuated glass can which turns viscous during hot isostatic pressing to hug the ceramic molding as a layer of glass.
With this known method for encapsulating the ceramic molding, the glass layer threatens to penetrate into the pores of the molding. Should the melting glass exhibit too low viscosity during hot isostatic pressing, the strength of the molding may be compromised at its surface zone. Another risk is that the glass, if of low viscosity, may penetrate into the pores of the molding to an extent where the glass envelope will intimately unite with the molding such that when the glass envelope is being removed after hot isostatic pressing, damage to the ceramic molding cannot be avoided especially if it is a complex, thin-walled shape. On the other hand there is the risk that the glass can, when melting during hot isostatic pressing, and the resulting glass layer hugging the ceramic moulding have excessive viscosity and deform the moulding, which should definitely be avoided.
U.S. Pat. No. 4,230,745 discloses a method of encapsulating a molded ceramic member wherein the molded member is first filled with N.sub.2, and then coated with a dense surface coating of Si melt and thereafter exposed to an N.sub.2 atmosphere until the Si coating is converted into a Si.sub.3 N.sub.4 coating. In contrast, the present invention provides a method of sealing a ceramic molding without forming a surface layer of glass to encapsulate the molding. The pores are sealed with plugs of Si.sub.3 N.sub.4. This improvement retains the original geometric shape, eliminates the possibility of a mistake in forming an external layer, and the possibility of trapping the metallic silicon residue between the coating and the molding.